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MRSA study flawed, resistance debate far from over

Last week researchers in England and Denmark claimed they had established the animal-to-human link of antimicrobial resistance. They used whole genome sequencing to “demonstrate that the MRSA strains we studied are capable of transmission between animals and humans,” according to the study’s lead author, Ewan Harrison.

Some scientists in the United States, however, claim the study is flawed. For instance, regarding bacterial genome sequencing, “It is irresponsible science to make conclusions that one came from another in the absence of some other information,” says Scott Hurd, associate professor and director of the Food Risk Modeling and Policy Laboratory at Iowa State University’s College of Veterinary Medicine.

Hurd wrote in his blog (www.hurdhealth.com) that the published study “outlines the zoonotic transmission of MRSA from animals to humans, based on genome sequencing. Although this is a possible route for MRSA, we could just as easily argue that people gave the genome to the animal strains of MRSA. Or better BOTH people and animals got these MRSA strains from a common bacterial source. Remember, we do all live in the same environmental, bacterial system.”

Following the release of the study from the England and Denmark researchers, Louise M. Slaughter, (D-N.Y.), claimed, “This study ends any debate. The extreme overuse of antibiotics in livestock is endangering human health.”

Slaughter’s claim would seem to be a stretch, especially considering the details of the European study. Specifically, two patients were studied – a 53-year-old woman and a 69-year-old woman. The first patient had two cows, two horses and a dog, and the second patient had a flock of 10 sheep.

Hurd, who served as Deputy Undersecretary for Food Safety at the U.S. Department of Agriculture, wrote that the exposure routes reported for the two women are very unique. The infections occurred in one case at the site of a steroid injection, and in the other case as a result of a serious chainsaw injury.

“The MRSA strain was susceptible to antibiotics other than beta lactam penicillins and both patients recovered. (There was NO harm, therefore NO risk),” Hurd wrote. “Because there is no discussion of antibiotic use on these farms, and considering their size, it is highly unlikely antibiotics were ever used in these animals and certainly not in the feed. These are very unique situations, which are informative, but CANNOT be used to make national policy. Remember that the science of risk assessment does not say that transfer of resistance NEVER occurs, but that it is too rare to justify major changes in animal antibiotic use policy. I don’t know why our friends in Denmark have a strong desire to ‘eradicate’ their livestock production, and then export those costly practices to the rest of us.”